For BusyBox, I recommend JRummy16’s installer but change the install location to /system/bin/ and choose “Advanced install.” The defaults on the advanced install page are fine. For whatever reason, the “Symlink applets” option does not work without going into advanced install.

Now in Terminal Emulator you can set “Command line” (your shell) from /system/bin/sh - to /system/bin/bash - and you’ll have most of your standard Linux commands.